Heaven Knows, Mr. Allison (1957)

In 1944, the castaway Corporal Allison, adrift in a raft since his ship was attacked, lands on tiny Tuasiva Island, where he meets Sister Angela. She tells him that she is the only person on the island, having been left behind while trying to evacuate a priest. The nun and the marine are stranded, but the island offers a bountiful supply of food. However, their paradisiacal life ends when the Japanese arrive to build an outpost, forcing "Mr." Allison and the nun to take refuge in a cave. The marine's expertise in such conditions proves to be vital to their survival, and the two grow ever closer.